Compact, fast zoom lens system

ABSTRACT

The present invention provides a zoom lens system comprising, in order from the object side, a first lens unit G1 having a positive refracting power; a second lens unit G2 including a front subunit having at least one negative lens and a rear subunit having at least one negative lens and at least two positive lenses located more closely to the image side than said negative lens and having a positive refracting power as a whole, said front and rear subunits having a positive composite power; and a third lens unit G3 having a negative refracting power; said first, second and third lens units including gaps between them, which are variable for zooming from the wide to tele position. This compact, three-unit zoom lens system has a zoom ratio of about 2 or more but, nonetheless, has a small F-number or is fast

BACKGROUND OF THE INVENTION

The present invention relates generally to a compact zoom lens systemsuitable for lens shutter cameras, etc., and more particularly to athree-unit or positive/positive/negative-unit, fast zoom lens systemwhich has a zoom ratio of ca. 2 and a small F-number.

In recent years, lens shutter cameras with built-in zoom lenses havebecome widely popular, and numerous zoom lenses suitable for lensshutter cameras, etc. have been proposed until now.

For instance, the zoom lenses proposed in Japanese Provisional PatentPublication Nos. 56-128911 and 57-201213 each comprise two or positiveand negative lens units. However, when it is intended to correct lensperformance well with a two-unit zoom lens, some difficulty isencountered in having a zoom ratio of 2 or more. Especially when it isintended to obtain a small F-number of ca. 3.5 for instance at the wideposition, the achievable zoom ratio is of the order of at most 1.5.

For this reason, a number of three-unit zoom lens systems have beenproposed as zoom lenses having a zoom ratio of 2 or more. According toJapanese Provisional Patent Publication Nos. Sho. 63-153351 and Hei.1-252916, zoom lenses having high zoom ratios are achieved byconstructing the optical systems of three or positive/positive/negativelens units.

In the three- or positive/positive/negative-unit zoom lenses proposed sofar in the art, however, the optical performance is maintained at theexpense of the F-number. When the focal length of the total system atthe tele position is of the order of 35 mm, the F-number is about 4.5,but this F-number is much more increased in association with zooming tothe tele position. In addition, when the focal length of the totalsystem is of the order of 80 mm, the F-number will assume a value of 6or more.

When it is intended to design a fast zoom lens having a small F-numberith the lens arrangement proposed so far in the art it is imperativelyrequired to diminish the refracting power of each lens. This gives riseto an excessive increase in the size of the lens system, failing toprovide a zoom lens suitable for use with a compact camera.

SUMMARY OF THE INVENTION

In view of the foregoing a main object of this invention is to provide acompact, fast, three-unit zoom lens system which has a zoom ratio of ca.2 but, nonetheless, has a small F-number.

According to one aspect of this invention, there is provided a compact,fast zoom lens system comprising, in order from the object side, a firstlens unit having a positive refracting power; a second lens unitincluding a front subunit having at least one negative lens and a rearsubunit having at least one negative lens and at least two positivelenses located more closely to the image side than said negative lensand having a positive refracting power as a whole, said front and rearsubunits having a positive composite power; and a third lens unit havinga negative refracting power;

said first, second and third lens units including gaps between them,which are variable for zooming from the wide to tele position.

According to another aspect of this invention, there is provided acompact, fast zoom lens system comprising, in order from the objectside, a first lens unit having a positive refracting power; a secondlens unit including a front subunit having at least one negative lensand a rear subunit having at least one negative lens and at least twopositive lenses located more closely to the image side than saidnegative lens and having a positive refracting power as a whole, saidfront and rear subunits having a positive composite power; and a thirdlens unit having a negative refracting power;

said first, second and third lens units including gaps between them,which are variable for zooming from the wide to tele position, and

further including a stop between said front and rear subunits.

Preferably, the lens of said rear subunit proximate to the image sidehas a positive refracting power and conforms to the following condition(1):

    0.5<(r.sub.A +r.sub.B)/(r.sub.A -r.sub.B)<2.0,             (1)

where

r_(A) is the radius of curvature on the object side of the lens of saidrear subunit of said second lens unit located proximately to the imageside, and

r_(B) is the radius of curvature on the image side of the lens of saidrear subunit of said second lens unit located proximately to the imageside

Preferably, the zoom lens system also conforms to the followingcondition (2):

    -0.55<f.sub.2R /f.sub.2F <0.2,                             (2)

where

f_(2F) is the focal length of said front subunit, and

f_(2R) is the focal length of said rear subunit.

In what follows, why the lens arrangement mentioned above is used andhow it acts will be explained.

As already mentioned, when it is intended to reduce the F-number of athree-unit or positive/positive/negative-unit zoom lens system, it isimperatively required to reduce the refracting power of each lens so asto secure performance. This incurs an increase in the size of the lenssystem. Especially in the case of this type zoom lens, the size of thesecond unit increases extremely, because the second unit is longer inthe total length and more in the number of lenses than the first andthird units.

In order to reduce the total length of the second unit, it is requiredto increase the refracting power of the rear subunit of the second unitand thereby narrow the space between the front and rear subunits of thesecond unit. In this case, a major aberrational problem arises inconnection with spherical aberration. This spherical aberration isgenerally balanced and corrected by plus and minus aberrations generatedby the front and rear subunits of the second unit, respectively.However, this correction has some difficulty, because the minusspherical aberration generated by the rear subunit is larger than theplus spherical aberration generated by the front subunit as much as therefracting power of the rear subunit is increased. Referring here to therear subunit of the second unit in more detail, it includes at least onenegative lens for correction of on-axial chromatic aberration and apositive lens in the rear thereof, by which some considerable sphericalaberration is generated. In this invention, the generation of thisspherical aberration is reduced by constructing that positive lens of atleast two positive lens components.

In addition, if a stop is located between the front and rear subunits ofthe second unit, then off axial aberrations are likely to be generatedby the lens of the rear subunit proximate to the image side, because onthe image side of the rear subunit off-axial luminous flux passesthrough a position off the optical axis. In the present invention, it isthus required that the lens of the rear subunit of the second unitproximate to the image side conform to the conditional formula (1)mentioned above.

When the lower limit of the conditional formula (1) is not reached, thesurface of that lens on the object side takes on a convex surface havinga small radius of curvature. Off-axial aberrations, esp., astigmatismgenerated by this surface are too large to correct. When the upper limitis exceeded, the image-side radius of curvature of the lens of the rearsubunit of the second unit proximate to the image side is so reducedthat this surface can generate spherical surface too large to correct.For these reasons, the conditional formula (1) should be satisfied.

As already stated, it is also desired that the present zoom lens systemconform to the conditional formula (2):

    -0.55<f.sub.2R /f.sub.2F <0.2,                             (2)

where f_(2F) and f_(2R) are the focal lengths of the front and rearsubunits of the second unit, respectively.

Falling short of the lower limit of the conditional formula (2) may bepreferred, because the refracting power of the front subunit of thesecond unit becomes negative and strong and the front and rear subunitsof the second unit are well balanced for correction of sphericalaberration. However, the inter-unit distance between the front and rearsubunits becomes too long. When the upper limit is exceeded, therefracting power of the front subunit of the second unit becomespositive, making it difficult to correct spherical aberration. Besides,since the principal point of the second unit is located so closely tothe object side that the interunit distance between the first and secondunits becomes wide, resulting in an increase in the total length of thezoom lens system.

It is noted that the incorporation of an aspherical surface in thesecond unit makes correction of spherical aberration easier and theperformance and total length of the zoom lens system much better andshorter. In addition, the performance of the zoom lens system is evenmuch more improved by incorporating aspherical surfaces in the first andthird units, thereby correcting off-axial aberrations in particular.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction set forth, and the scope of theinvention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) represent in section the zoom lens system of Example1 according to this invention at the (a) wide and (b) tele positions,

FIG. 2 represents in section the zoom lens system of Example 2 at thewide position,

FIG. 3 represents in section the zoom lens system of Example 3 at thewide position,

FIG. 4 represents in section the zoom lens system of Example 4 at thewide position,

FIG. 5 represents in section the zoom lens system of Example 5 at thewide position,

FIGS. 6(a)-6(c) are aberration diagrams showing the sphericalaberration, astigmatism, distortion and chromatic aberration ofmagnification of Example 1 at the (a) wide, (b) standard and (c) telepositions.

FIGS. 7(a)-7(c) are aberration diagrams of Example 2, similar to FIGS.6(a)-6(c),

FIGS. 8(a)-8(c) are aberration diagrams of Example 3, similar to FIGS.6(a)-6(c),

FIGS. 9(a)-9(c) are aberration diagrams of Example 4, similar to FIGS.6(a)-6(c),

FIGS. 10(a)-10(c) are aberration diagrams of Example 5, similar to FIGS.6(a)-6(c).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, some examples of the zoom lens accordingto this invention will be explained more specifically.

Note that the lens data of the examples will be given later. Thesections of the lens arrangement of Example 1 at the (a) wide and (b)tele positions are shown in FIG. 1, and the sections of the lensarrangements of Examples 2-5 at the wide positions are shown in FIGS.2-5, respectively.

Referring to the lens arrangement of each unit, the first unit G1comprises in Example 1 a double-concave negative lens and two positivemeniscus lenses, three in all; in Example 2 or 3 a cemented lens of adouble concave negative lens and a positive meniscus lens and a positivemeniscus lens, three in all; in Example 4 a double-concave negativelens, a positive meniscus lens and a double-convex lens, three in all;and in Example 5 a double-concave lens and two double-convex positivelenses, three in all.

In each example, the front subunit G2F of the second unit G2 comprises adouble-concave negative lens and a double-convex positive lens, two inall. The rear subunit G2R of the second unit G2 comprises in Example 1 adouble-convex positive lens, a cemented lens of a double-concavenegative lens and a double-convex positive lens and a positive meniscuslens, four in all; in Example 2 or 3 a positive meniscus lens, acemented lens of a double-concave negative lens and a double-convexpositive lens and a positive meniscus lens, four in all; in Example 4 acemented lens of a double-concave negative lens and a double-convexpositive lens and a double-convex positive lens, three in all; and inExample 5 a cemented lens of a double-concave negative lens and a doubleconvex positive lens, a double-convex positive lens and a positivemeniscus lens, four in all.

The third unit G3 comprises in Example 1 or 5 a positive meniscus lensand two negative meniscus lenses, three in all; in Example 2 a positivemeniscus lens and a double-convex negative lens, two in all: in Example3 a positive meniscus lens, a negative meniscus lens and adouble-concave negative lens, three in all; and in Example 4 a positivemeniscus lens, a double-concave negative lens and a negative meniscuslens, three in all. Thus, the zoom lens system of Example 1, 3 or 5comprises 12 lenses in all and that of Examples 2 or 4 comprises 11lenses in all.

In each example, one aspherical surface is applied to the image-sidesurface of the positive lens of the rear subunit G2R of the second unitproximate to the object side.

It is noted that the symbols used hereinafter but not referred tohereinbefore mean:

f: focal length of the total system

F_(NO) : F-number

ω: half field angle

f_(B) : back focus

r₁, r₂, . . . : radii of curvature of the lens surfaces

d₁, d₂, . . . : separation between adjacent lens surfaces

n_(d1), n_(d2), . . . : d-line refractive indices of the lenses

ν_(d1), ν_(d2), . . . : Abbe's number of the lenses

Here let x denote the optical axis direction and y denote the directionnormal to the optical axis, then aspherical configuration is expressedby the following formula:

    x=Cy.sup.2 /{1+(1-(C.sup.2 y.sup.2).sup.1/2 }+A.sub.4 y.sup.4 +A.sub.6 y.sup.6 +A.sub.8 y.sup.8 +A.sub.10 y.sup.10,

provided that C=1/r, r is the radius of curvature on the optical axisand A₄, A₆, A₈ and A₁₀ are aspher1cal coefficients.

EXAMPLE 1

    ______________________________________                                        f = 36.0˜ 49.5˜ 68.2                                              F.sub.NO = 2.88˜ 3.22˜ 3.60                                       ω = 32.08˜ 23.35˜ 17.18°                             f.sub.B = 7.08˜ 16.1˜ 27.54                                       r.sub.1 = -800.000                                                                        d.sub.1 = 2.400                                                                           n.sub.d1 = 1.68893 ν.sub.d1 = 31.08                r.sub.2 = 61.241                                                                          d.sub.2 = 0.255                                                   r.sub.3 = 66.830                                                                          d.sub.3 = 4.600                                                                           n.sub.d2 = 1.60311 ν.sub.d2 = 60.70                r.sub.4 = 1601.373                                                                        d.sub.4 = 0.200                                                   r.sub.5 = 47.535                                                                          d.sub.5 = 4.500                                                                           n.sub.d3 = 1.65830 ν.sub.d3 = 57.33                r.sub.6 = 2166.091                                                                        d.sub.6 = (Variable)                                              r.sub.7 = -53.803                                                                         d.sub.7 = 1.300                                                                           n.sub.d4 = 1.77250 ν.sub.d4 = 49.66                r.sub.8  = 18.861                                                                         d.sub.8 = 0.750                                                   r.sub.9 = 22.986                                                                          d.sub.9 = 4.916                                                                           n.sub.d5 = 1.80518 ν.sub.d5 = 25.43                r.sub.10 = -773.166                                                                       d.sub.10 = 1.500                                                  r.sub.11 = ∞ (Stop)                                                                 d.sub.11 = 4.552                                                  r.sub.12 = 55.930                                                                         d.sub.12 = 5.000                                                                          n.sub.d6 = 1.58913 ν.sub.d6 = 61.18                r.sub.13 = -29.993                                                                        d.sub.13 = 1.600                                                  (Aspheric)                                                                    r.sub.14 = -20.326                                                                        d.sub.14 = 2.312                                                                          n.sub.d7 = 1.78472 ν.sub.d7 = 25.68                r.sub.15 = 64.526                                                                         d.sub.15 = 5.494                                                                          n.sub.d8 = 1.69680 ν.sub.d8 = 55.52                r.sub.16 = -20.043                                                                        d.sub.16 = 0.200                                                  r.sub.17 = -156.451                                                                       d.sub.17 = 3.011                                                                          n.sub.d9 = 1.60311 ν.sub.d9 = 60.70                r.sub.18 = -32.017                                                                        d.sub.18 = (Variable)                                             r.sub.19 = -55.646                                                                        d.sub.19 = 3.500                                                                          n.sub.d10 = 1.80518 ν.sub.d10 = 25.43              r.sub.20 = -26.780                                                                        d.sub.20 = 2.463                                                  r.sub.21 = -28.784                                                                        d.sub.21 = 1.800                                                                          n.sub.d11 = 1.69680 ν.sub.d11 = 55.52              r.sub.22 = -660.853                                                                       d.sub.22 = 4.960                                                  r.sub.23 = -22.140                                                                        d.sub.23 = 2.100                                                                          n.sub.d12 = 1.72916 ν.sub.d12 = 54.68              r.sub.24 = -112.630                                                           Zooming Speces                                                                f   36.0    49.5    68.2                                                      d.sub.6                                                                           4.600   14.803  23.671                                                    d.sub.18                                                                          11.341  6.164   1.614                                                     Aspherical Coefficients                                                       13th surface                                                                  A.sub.4 = 2.7011 × 10.sup.-5                                            A.sub.6 = 5.3504 × 10.sup.-9                                            A.sub.8 = 1.0223 × 10.sup.-9                                            A.sub.10 = -3.5508 × 10.sup.-12                                         (r.sub.A + r.sub.B)/(r.sub.A - r.sub.B) = 1.51                                f.sub.2R /f.sub.2F = -0.45                                                    ______________________________________                                    

EXAMPLE 2

    ______________________________________                                        f = 36.2˜ 52.9˜ 77.2                                              F.sub.NO = 2.90˜ 3.49˜ 4.60                                       ω = 31.88˜ 21.58˜ 15.16°                             f.sub.B = 8.31˜ 20.05˜ 40.40                                      r.sub.1 = ∞                                                                         d.sub.1 = 2.500                                                                           n.sub.d1 = 1.78472 ν.sub.d1 = 25.68                r.sub.2 = 67.225                                                                          d.sub.2 = 4.100                                                                           n.sub.d2 = 1.61405 ν.sub.d2 = 54.95                r.sub.3 = 237.641                                                                         d.sub.3 = 0.200                                                   r.sub.4 = 34.228                                                                          d.sub.4 = 4.800                                                                           n.sub.d3 = 1.69680 ν.sub.d3 = 56.49                r.sub.5 = 407.736                                                                         d.sub.5 = (Variable)                                              r.sub.6 = -47.492                                                                         d.sub.6 = 1.100                                                                           n.sub.d4 = 1.72916 ν.sub.d4 = 54.68                r.sub.7 = 16.774                                                                          d.sub.7 = 1.601                                                   r.sub.8  = 24.127                                                                         d.sub.8 = 3.500                                                                           n.sub.d5 = 1.80518 ν.sub.d5 = 25.43                r.sub.9 = -95.068                                                                         d.sub.9 = 2.000                                                   r.sub.10 = ∞ (Stop)                                                                 d.sub.10 = 2.000                                                  r.sub.11 = -149.985                                                                       d.sub.11 = 3.500                                                                          n.sub.d6 = 1.58913 ν.sub.d6 = 61.68                r.sub.12 = -28.924                                                                        d.sub.12 = 1.200                                                  (Aspheric)                                                                    r.sub.13 = -24.694                                                                        d.sub.13 = 1.600                                                                          n.sub.d7 = 1.78472 ν.sub.d7 = 25.68                r.sub.14 = 34.573                                                                         d.sub.14 = 5.300                                                                          n.sub.d8 = 1.61272 ν.sub.d8 = 58.75                r.sub.15 = -26.622                                                                        d.sub.15 = 0.200                                                  r.sub.16 = -76.103                                                                        d.sub.16 = 4.000                                                                          n.sub.d9 = 1.69680 ν.sub.d9 = 56.49                r.sub.17 = -19.077                                                                        d.sub.17 =  (Variable)                                            r.sub.18 = -33.416                                                                        d.sub.18 = 3.242                                                                          n.sub.d10 = 1.78472 ν.sub.d10 = 25.68              r.sub.19 = -24.560                                                                        d.sub.19 = 3.839                                                  r.sub.20 = -25.011                                                                        d.sub.20 = 2.100                                                                          n.sub.d11 = 1.72916 ν.sub.d11 = 54.68              r.sub.21 = 96.031                                                             Zooming Speces                                                                f   36.2    52.9    77.2                                                      d.sub.5                                                                           3.500   14.032  18.789                                                    d.sub.17                                                                          17.509  9.651   2.500                                                     Aspherical Coefficients                                                       12th surface                                                                  A.sub.4 = 4.2078 × 10.sup.-5                                            A.sub.6 = 4.7849 × 10.sup.-8                                            A.sub.8 = 2.0427 × 10.sup.-9                                            A.sub.10 = -1.8664 × 10.sup.-11                                         (r.sub.A + r.sub.B)/(r.sub.A - r.sub.B) = 1.67                                f.sub.2R /f.sub.2F = -0.37                                                    ______________________________________                                    

EXAMPLE 3

    ______________________________________                                        f = 36.2˜ 52.9˜ 77.2                                              F.sub.NO = 2.90˜ 3.55˜ 4.73                                       ω = 31.26˜ 21.5˜ 15.18°                              f.sub.B = 8.29˜ 18.0˜ 34.69                                       r.sub.1 = ∞                                                                         d.sub.1 = 2.200                                                                           n.sub.d1 = 1.78472 ν.sub.d1 = 25.68                r.sub.2 = 85.519                                                                          d.sub.2 = 3.700                                                                           n.sub.d2 = 1.58913 ν.sub.d2 = 61.18                r.sub.3 = 2436.379                                                                        d.sub.3 = 0.200                                                   r.sub.4 = 35.920                                                                          d.sub.4 = 4.500                                                                           n.sub.d3 = 1.56873 ν.sub.d3 = 63.16                r.sub.5 = 574.778                                                                         d.sub.5 = (Variable)                                              r.sub.6 = -49.356                                                                         d.sub.6 = 1.100                                                                           n.sub.d4 = 1.72916 ν.sub.d4 = 54.68                r.sub.7 = 18.495                                                                          d.sub.7 = 2.027                                                   r.sub.8  = 24.760                                                                         d.sub.8 = 2.500                                                                           n.sub.d5 = 1.80518 ν.sub.d5 = 25.43                r.sub.9 = -151.234                                                                        d.sub.9 = 2.000                                                   r.sub.10 = ∞ (Stop)                                                                 d.sub.10 = 2.000                                                  r.sub.11 = -567.531                                                                       d.sub.11 = 4.000                                                                          n.sub.d6 = 1.53172 ν.sub.d6 = 48.90                r.sub.12 = -27.759                                                                        d.sub.12 = 1.432                                                  (Aspheric)                                                                    r.sub.13 = -19.077                                                                        d.sub.13 = 1.600                                                                          n.sub.d7 = 1.80518 ν.sub.d7 = 25.43                r.sub.14 = 58.976                                                                         d.sub.14 = 5.080                                                                          n.sub.d8 = 1.60311 ν.sub.d8 = 60.70                r.sub.15 = -19.015                                                                        d.sub.15 = 0.200                                                  r.sub.16 = -242.238                                                                       d.sub.16 = 4.285                                                                          n.sub.d9 = 1.61700 ν.sub.d9 = 62.79                r.sub.17 = -20.359                                                                        d.sub.17 =  (Variable)                                            r.sub.18 = -61.608                                                                        d.sub.18 = 3.200                                                                          n.sub.d10 = 1.71736 ν.sub.d10 = 29.51              r.sub.19 = -26.124                                                                        d.sub.19 = 1.731                                                  r.sub.20 = -34.091                                                                        d.sub.20 = 1.800                                                                          n.sub.d11 = 1.72916 ν.sub.d11 = 54.68              r.sub.21 = -340.771                                                                       d.sub.21 = 4.004                                                  r.sub.22 = -21.489                                                                        d.sub.22 = 2.100                                                                          n.sub.d12 = 1.69680 ν.sub.d12 = 55.52              r.sub.23 = 912.716                                                            Zooming Speces                                                                f   36.2    52.9    77.2                                                      d.sub.5                                                                           4.500   15.315  20.235                                                    d.sub.17                                                                          12.623  7.026   2.000                                                     Aspherical Coefficients                                                       12th surface                                                                  A.sub.4 = 5.5788 × 10.sup.-5                                            A.sub.6 = 1.8165 × 10.sup.-7                                            A.sub.8 = 2.0219 × 10.sup.-10                                           A.sub.10 = 6.2863 × 10.sup.-12                                          (r.sub.A + r.sub.B)/(r.sub.A - r.sub.B) = 1.18                                f.sub.2R /f.sub.2F = -0.32                                                    ______________________________________                                    

EXAMPLE 4

    ______________________________________                                        f = 28.95˜ 44.32˜ 68.19                                           F.sub.NO = 3.60˜ 4.10˜ 4.63                                       ω = 38.46˜ 25.29˜ 17.0°                              f.sub.B = 6.64˜ 21.74˜ 42.66                                      r.sub.1 = -800.000                                                                        d.sub.1 = 2.300                                                                           n.sub.d1 = 1.83400 ν.sub.d1 = 37.16                r.sub.2 = 32.873                                                                          d.sub.2 = 2.164                                                   r.sub.3 = 36.376                                                                          d.sub.3 = 5.327                                                                           n.sub.d2 = 1.51633 ν.sub.d2 = 64.15                r.sub.4 = 7892.397                                                                        d.sub.4 = 0.200                                                   r.sub.5 = 46.254                                                                          d.sub.5 = 5.680                                                                           n.sub.d3 = 1.62230 ν.sub.d3 = 53.20                r.sub.6 = -97.767                                                                         d.sub.6 = (Variable)                                              r.sub.7 = -76.115                                                                         d.sub.7 = 1.300                                                                           n.sub.d4 = 1.72916 ν.sub.d4 = 54.68                r.sub.8 = 14.973                                                                          d.sub.8 = 3.315                                                   r.sub.9 = 22.917                                                                          d.sub.9 = 3.600                                                                           n.sub.d5 = 1.76182 ν.sub.d5 = 26.52                r.sub.10 = -58.481                                                                        d.sub.10 = 3.000                                                  r.sub.11 = ∞ (Stop)                                                                 d.sub.11 = 6.608                                                  r.sub.12 = -26.430                                                                        d.sub.12 = 1.214                                                                          n.sub.d6 = 1.78472 ν.sub.d6 = 25.68                r.sub.13 = 23.375                                                                         d.sub.13 = 5.341                                                                          n.sub.d7 = 1.58913 ν.sub.d7 = 61.18                r.sub.14 = -23.179                                                                        d.sub.14 = 0.200                                                  (Aspheric)                                                                    r.sub.15 = 69.729                                                                         d.sub.15 = 6.291                                                                          n.sub.d8 = 1.56873 ν.sub.d8 = 63.16                r.sub.16 = -19.592                                                                        d.sub.16 = (Variable)                                             r.sub.17 = -46.811                                                                        d.sub.17 = 3.500                                                                          n.sub.d9 = 1.80518 ν.sub.d9 = 25.43                r.sub.18 = -27.290                                                                        d.sub.18 = 0.200                                                  r.sub.19 = -83.026                                                                        d.sub.19 = 1.880                                                                          n.sub.d10 = 1.77250 ν.sub.d10 = 49.66              r.sub.20 = 334.950                                                                        d.sub.20 = 6.427                                                  r.sub.21 = -19.299                                                                        d.sub.21 = 2.100                                                                          n.sub.d11 = 1.72916 ν.sub.d11 = 54.68              r.sub.22 = -112.630                                                           Zooming Speces                                                                f   28.95   44.32   68.19                                                     d.sub.6                                                                           3.000   14.614  24.952                                                    d.sub.16                                                                          15.249  7.588   1.614                                                     Aspherical Coefficients                                                       14th surface                                                                  A.sub.4 = 2.2122 × 10.sup.-5                                            A.sub.6 = 7.3888 × 10.sup.-8                                            A.sub.8 = 4.4143 × 10.sup.-10                                           A.sub.10 = -2.0132 × 10.sup.-12                                         (r.sub.A + r.sub.B)/(r.sub.A - r.sub.B) = 0.56                                f.sub.2R /f.sub.2F = -0.07                                                    ______________________________________                                    

EXAMPLE 5

    ______________________________________                                        f = 29.10˜ 44.31˜ 68.17                                           F.sub.NO = 3.60˜ 4.10˜ 4.63                                       ω = 38.46˜ 25.4˜ 16.93°                              f.sub.B = 6.45˜ 20.97˜ 39.35                                      r.sub.1 = -800.000                                                                        d.sub.1 = 2.300                                                                           n.sub.d1 = 1.83400 ν.sub.d1 = 37.16                r.sub.2 = 36.570                                                                          d.sub.2 = 2.099                                                   r.sub.3 = 39.897                                                                          d.sub.3 = 5.510                                                                           n.sub.d2 = 1.51633 ν.sub.d2 = 64.15                r.sub.4 = -597.969                                                                        d.sub.4 = 0.200                                                   r.sub.5 = 49.018                                                                          d.sub.5 = 5.558                                                                           n.sub.d3 = 1.62230 ν.sub.d3 = 53.20                r.sub.6 = -117.434                                                                        d.sub.6 = (Variable)                                              r.sub.7 = -52.214                                                                         d.sub.7 = 1.300                                                                           n.sub.d4 = 1.72916 ν.sub.d4 = 54.68                r.sub. 8 = 16.799                                                                         d.sub.8 = 3.260                                                   r.sub.9 = 23.685                                                                          d.sub.9 = 3.647                                                                           n.sub.d5 = 1.78472 ν.sub.d5 = 25.68                r.sub.10 = -43.345                                                                        d.sub.10 = 1.825                                                  r.sub.11 = ∞ (Stop)                                                                 d.sub.11 = 5.801                                                  r.sub.12 = -19.169                                                                        d.sub.12 = 1.308                                                                          n.sub.d6 = 1.78472 ν.sub.d6 = 25.68                r.sub.13 = 20.527                                                                         d.sub.13 = 5.278                                                                          n.sub.d7 = 1.58913 ν.sub.d7 = 61.18                r.sub.14 = -24.131                                                                        d.sub.14 = 0.200                                                  (Aspheric)                                                                    r.sub.15 = 105.845                                                                        d.sub.15 = 5.156                                                                          n.sub.d8 = 1.51633 ν.sub.d8 = 64.15                r.sub.16 = -21.946                                                                        d.sub.16 = 0.199                                                  r.sub.17 = -76.006                                                                        d.sub.17 = 3.707                                                                          n.sub.d9 = 1.51633 ν.sub.d9 =  64.15               r.sub.18 = -23.836                                                                        d.sub.18 = (Variable)                                             r.sub.19 = -44.045                                                                        d.sub.19 = 3.500                                                                          n.sub.d10 = 1.80518 ν.sub.d10 = 25.43              r.sub.20 = -25.625                                                                        d.sub.20 = 0.200                                                  r.sub.21 = -60.581                                                                        d.sub.21 = 1.880                                                                          n.sub.d11 = 1.77250 ν.sub.d11 = 49.66              r.sub.22 = -2605.015                                                                      d.sub.22 = 6.460                                                  r.sub.23 = -18.782                                                                        d.sub.23 = 2.100                                                                          n.sub.d12 = 1.72916 ν.sub.d12 = 54.68              r.sub.24 = -112.630                                                           Zooming Speces                                                                f   29.10   44.31   68.17                                                     d.sub.6                                                                           3.000   14.198  26.490                                                    d.sub.18                                                                          14.506  7.206   1.614                                                     Aspherical Coefficients                                                       14th surface                                                                  A.sub.4 = 2.2286 × 10.sup.-5                                            A.sub.6 = 9.7844 × 10.sup.-8                                            A.sub.8 = 2.9158 × 10.sup.-10                                           A.sub.10 =  4.8489 × 10.sup.-12                                         (r.sub.A + r.sub.B)/(r.sub.A - r.sub.B) = 1.91                                f.sub.2R /f.sub.2F = -0.13                                                    ______________________________________                                    

The spherical aberrations, astigmatisms, distortions and chromaticaberrations of magnification of Examples 1-5 at the (a) wide, (b)standard and (c) tele positions are shown in the aberration diagrams ofFIGS. 6(a)-10(c).

As explained above, the present invention provides a fast zoom lenssystem which has a zoom ratio of 2 or more and a small F-number and ismade compact but, nonetheless, is of high performance.

What we claim is:
 1. A zoom lens system comprising, in order from theobject side:a first lens unit having a positive refractive power; asecond lens unit including:a front subunit having at least one negativelens; and a rear subunit having at least one negative lens and at leasttwo positive lenses, wherein said at least two positive lenses aredisposed closer to the image side than said negative lens of the rearsubunit and wherein said rear subunit has a positive refracting power asa whole, said front and rear subunits having a positive composite power;and a third lens unit having a negative refracting power; said first,second and third lens units having spaces therebetween, said spacesbeing variable while the space between said front and rear subunitsremains fixed during zooming from the wide-angle end to the telephotoend; and further including a stop between said front and said rearsubunits.
 2. A zoom lens system as claimed in claim 1, wherein the lensof said rear subunit proximate to the image side has a positiverefracting power and satisfies the following condition (1):

    0.5<(r.sub.A +r.sub.B)/(r.sub.A -r.sub.B)<2.0,             (1)

where r_(A) is the radius of curvature on the object side of the lens ofsaid rear subunit of said second lens unit located proximately to theimage side, and r_(B) is the radius of curvature on the image side ofthe lens of said rear subunit of said second lens unit locatedproximately to the image side.
 3. A zoom lens system as claimed in claim1, which satisfies the following condition (2):

    -0.55<f.sub.2R /f.sub.2R <0.2,                             (2)

where f_(2F) is the focal length of said front subunit, and f_(2R) isthe focal length of said rear subunit.